1. Field of the Invention
The present invention generally relates to a deflection yoke in a cathode ray tube used in a television receiver set and, more particularly, to the deflection yoke of a type mounted on a color cathode ray tube in the vicinity of the funnel and neck sections thereof for deflecting election beams emitted from the electron gun assembly in the cathode ray tube.
2. Description of the Prior Art
Some prior art deflection yokes which appear to be pertinent to the present invention will be discussed with reference to some of the accompanying drawings.
Referring first to FIG. 19 showing, in schematic longitudinal sectional representation, a color cathode ray tube utilizing the prior art deflection yoke, the color cathode ray tube shown therein and generally identified by 50 comprises a highly evacuated envelope including a generally conical funnel section 52, a generally cylindrical neck section 53 protruding from a reduced diameter end of the funnel section 52 and a faceplate 51 provided at a large diameter end of the funnel section 52 opposite the neck section 53. The faceplate 51 has its inner surface deposited with a phosphor coating 55 thereby defining an effective screen region 100 of the faceplate 51, at least a portion of the faceplate 51 where the phosphor coating 55 is deposited being generally concaved so as to protrude outwardly in a direction away from the neck section 53. The cathode ray tube includes an electron gun assembly 54 disposed inside the neck section 53 and a finely perforated shadow mask 56 positioned inside the faceplate 51 and spaced a predetermined distance therefrom so as to extend generally parallel to the faceplate 51. As is well known to those skilled in the art, the shadow mask 56 is made of a metal thin plate having a predetermined pattern of fine apertures 56a which are generally triads of minute circular holes and is so curved as to follow the curvature of the faceplate 51. The shadow mask 56 has its peripheral edge secured to a support frame member 57 which is retained in position inside the faceplate 51 by means of a retainer not shown in equally spaced relation to the faceplate 51.
The deflection yoke is generally identified by 1 and is mounted externally on the cathode ray tube 50 at the boundary between the funnel section 52 and the neck section 53.
In the cathode ray tube 50 of the above described construction, electron beams 70 emitted from the electron gun assembly 54 travel towards the faceplate 51 through the perforations 56a of the shadow mask 56 and impinge upon the effective screen region 100, that is, the phosphor-coated screen. The electron beams 70 then traveling from the electron gun assembly 54 towards the phosphor-coated screen 100 pass through an electromagnetic field developed inside the cathode ray tube 50 by the deflection yoke 1. During the passage of the electron beams 70 through the electromagnetic field, the electron beams 70 are deflected at a deflection center in the effective electromagnetic field, which effective center is indicated by D, and, after having passed through the fine apertures 56a of the shadow mask 56, impinge upon predetermined areas of the phosphor coating 55 thereby to cause such predetermined areas of the phosphor coating 55 to emit light.
The details of the prior art deflection yoke 1 are illustrated in FIG. 20 in perspective representation as viewed from rear, i.e., as viewed from the neck section 53 towards the funnel section 52. As shown, the deflection yoke 1 comprises a generally tubular core 2 made of ferromagnetic material such as, for example, ferrite, a horizontal deflection coil assembly 3 and a vertical deflection coil assembly 4. The horizontal deflection coil assembly 3 comprises a pair of generally saddle-shaped coils wound on the core 2 and arranged in opposite and symmetrical relation to each other with respect to the longitudinal axis of the tubular core 2 and is operable to develop an electromagnetic field necessitated to deflect the electron beams 70 is a horizontal direction. The vertical deflection coil assembly 3 similarly comprises a pair of generally toroidally wound coils on the core 2 and arranged in opposite and symmetrical relation to each other with respect to the longitudinal axis of the tubular core 2 and is operable to develop an electromagnetic field necessitated to deflect the electron beams 70 in a vertical direction perpendicular to the horizontal direction.
In the cathode ray tube of such a construction as hereinbefore described, there is a problematic tendency on the part of the shadow mask 56 to produce a localized buckling or "doming". This doming is a phenomenon in which a localized heating of the shadow mask 56, generally convexed in a direction conforming to the direction of travel of the electron beams 70, takes place as a result of bombardment of the electron beams 70, then traveling towards the phosphor-coated screen region 100. As a result those areas of the shadow mask 56 where heat concentration takes place undergo a thermal expansion to cause those areas of the shadow mask 56 to further protrude outwardly with respect to the direction of travel of the electron beams 70, that is, in a direction close towards the phosphor-coated screen region 100. Once this doming occurs, those areas of the shadow mask 56 displace themselves from their original positions to different positions in which the shadow mask 56 does not effect proper masking of the incoming electron beams 70, and mislanding of the electron beams 70 upon the phosphor-coated screen region 100 takes place resulting in undesirable degradation of color purity.
As one of the countermeasures for eliminating the doming, a bulletin entitled "Terebijon (Television)", Vol. 31, No. 6, pages 46 to 52, describes that the radius of curvature of the shadow mask should be as small as possible.
The curvature of the shadow mask 56 is generally selected in consideration of the curvature of the phosphor-coated screen region 100 on the faceplate 51 and, therefore, the selection of the radius of curvature of the phosphor-coated screen region 100 on the faceplate 51 to a value as small as possible is an effective means for reducing the radius of curvature of the shadow mask 56.
However, the selection of the smaller radius of curvature of the phosphor-coated screen region 100 results in the design of a cathode ray tube in which both of the phosphor-coated screen region 100 and the shadow mask 56 which are generally convexed so as to protrude in a direction towards a television viewer are further protruded towards the television viewer. This design of the cathode ray tube is in contrast to the recent trend in which the faceplate is designed generally flat so as to render a televised picture to be comfortable to view.
In view of the recent trend, a compromise has been made to provide a cathode ray tube having its faceplate rendered to be of non-spherical shape, the faceplate of non-spherical shape being hereinafter referred to as an SP panel.
Hereinafter, features of the SP panel and problems associated with picture distortion peculiar to the cathode ray tube having the SP panel will be discussed.
For the purpose of discussion, FIG. 21 illustrates the generally rectangular phosphor-coated screen region 100 taken as a system of Cartesian coordinates wherein the point of origin 0 lies at the center of the phosphor-coated screen region 100 in alignment with the longitudinal axis of the cathode ray tube 50 while the x-axis extends in a direction parallel to the longer side of the rectangular shape of the phosphor-coated screen region 100, the y-axis extends in a direction parallel to the shorter side of the same and the z-axis extends in a direction in which the phosphor-coated screen region 100 protrudes. The x-axis and y-axis correspond to the horizontal and vertical deflecting direction of the deflection yoke 1 discussed with reference to FIG. 19, respectively.
FIG. 22 illustrates a relative positional relationship between the deflection yoke 1 and the phosphor-coated screen region 100, and FIG. 23 represents a schematic section of one of the halves of the phosphor-coated screen region 100 taken in a plane containing the x-axis and y-axis. Assuming that the section of the phosphor-coated screen region 100 is expressed by z=f(x), the radius of curvature Px when x is an arbitrarily chosen value can be expressed by the following equation: EQU P x=-[1+(df/dx).sup.2 ].sup.3/2 /(d.sup.2 f/dx.sup.2)
Thus when the radius of curvature Px takes a positive value, the phosphor-coated screen region 100 protrudes in a direction conforming to the +z-axis.
Assuming that the distance from the point of origin 0 to one extremity of the phosphor-coated screen region 100 in the x-axis direction is expressed by Xmax, it can be said that the SP panel has a portion where the radius of curvature smaller than the radius of curvature PO at x=0, which portion is located between locations spaced respective distances 2/3 Xmax and 3/4 Xmax away from the point of origin 0 in the plane containing the x-axis and y-axis.
In a cathode ray tube having the above discussed SP panel, the occurrence of the doming can be advantageously minimized without adversely affecting the flatness of the screen as a whole because of the reason stated below.
Specifically, even though the doming takes place considerably at that area of the shadow mask 56 where the angle of deflection is small, that is, which is close to the center (x=0) of the phosphor-coated screen region 100, the displacement of that area of the shadow mask 56 brings about small mislanding of the electron beams 70, and therefore brings about less harm, and the flatness in the vicinity of the center of the phosphor-coated screen region 100 is important for the SP panel to be viewed as flat. Therefore, it is desirable for the radius of curvature Po to take a relatively great value.
At the periphery of the phosphor-coated screen region 100 spaced the distance Xmax from the center thereof, since the peripheral edge of the shadow mask 56 is fixed to the support frame member 57, the peripheral portion of the shadow mask 56 is substantially free from thermal deformation resulting from the doming, or if it occurs, the sight of a television viewer is seldom centered on the peripheral portion of the SP panel and, therefore, the television viewer will not be adversely affected. In view of this, the radius of curvature P at x=Xmax may be considered having no weight.
Summarizing the above, it can be concluded that, in order to minimize the occurrence of the doming and the attendant reduction in color purity, the radius of curvature Px at a portion of the phosphor-coated screen region 100 between x=0 and x=Xmax, particularly between x=2/3 and x=3/4 Xmax, should be as small as possible as compared with the radius of curvature Po at the center of the phosphor-coated screen region 100. The SP panel is a product developed on the basis of this conclusion.
Another problem associated with distortion of the televised picture being reproduced will now be discussed. When, for example, an image of horizontal parallel lines are reproduced on the screen of the cathode ray tube having the generally spherical phosphor-coated screen region 100, the result would be such as shown in FIG. 24. More specifically, while the horizontal lines 110 forming the image ought to have been reproduced parallel to each other as depicted by the dotted lines in FIG. 24, the horizontal lines 110 actually reproduced are so curved as to diverge away from the x-axis as the distance in the x-axis direction increases away from the center of the phosphor-coated screen region 100. This is because the distance between the deflection point D in the electromagnetic field developed by the deflection yoke 1 and the phosphor-coated screen region 100 shown in FIG. 19 increases with increase of the distance away from the center of the phosphor-coated screen region 100 in the x-axis direction.
Assuming that the magnitude of the divergence of the horizontal lines 110 away from the x-axis with increase of the distance away from the center of the phosphor-coated screen region 100 in the x-axis direction is expressed by .DELTA. y, the magnitude of divergence .DELTA. y is substantially proportional to x.sup.2 y when the point of interest of the phosphor-coated screen region 100 in the system of Cartesian coordinates is (x, y). Since the phosphor-coated screen region 100 is generally spherical in shape, the magnitude of divergence .DELTA. y can be reduced to a certain extent if the radius of curvature of the phosphor-coated screen region 100 is reduced. Also, the magnitude of divergence .DELTA. y can also be reduced to a certain extent if the pattern of the electromagnetic field developed by the deflection yoke 1 is suitably tailored.
Although with the combination of the reduced radius of curvature of the screen region 100 and the tailored pattern of the electromagnetic field, complete removal of the above distortion of the horizontal lines 110 is difficult and a small divergence .DELTA. y remains, the magnitude of remaining divergence .DELTA. y is still proportional to the value of x.sup.2 y or simple function of second order of x and first order of y and, therefore, the addition of a distortion correcting circuit to the deflecting circuit makes it possible to substantially eliminate the distortion.
When it comes to the cathode ray tube having the SP panel, the same horizontal lines 110 will be reproduced on the screen thereof as schematically illustrated in FIG. 25. In the SP panel, since the radius of curvature of the phosphor-coated screen region 100 in a plane containing both of the x-axis and z-axis is such as hereinbefore discussed, the amount of change in distance between the deflection point D and the phosphor-coated screen region 100 differs at respective portions inwardly and outwardly with respect to the location where x=2/3 Xmax in the plane containing both of the x-axis and y-axis and, therefore, not only is the magnitude of divergence .DELTA. y proportional to the value x.sup.2 y, but also the horizontal lines 110 diverging away from the x-axis tend to suddenly approach the x-axis at a portion outward of 2/3 Xmax away from the center of the phosphor-coated screen region 100 (X&gt;2/3 Xmax).
When viewed in a cross-section of one longer side portion (i.e., at a location of y=Ymax) of the SP panel taken along a plane parallel to the plane containing both of the x-axis and z-axis, as one of the means by which the screen as a whole can be considered flat, it may be contemplated to reduce the width of change of the phosphor-coated screen region 100 in the z-axis direction with the consequence that the bending of the horizontal lines 110 towards the x-axis can be lessened. Where this method is employed, the horizontal lines 110 reproduced on the screen would be such as shown in FIG. 26.
The distortion such as shown in FIG. 26 is referred to as a seagull distortion. The use of a distortion correcting circuit in the deflection circuit appears to be effective to substantially eliminate the occurrence of the seagull distortion. However, the distortion correcting circuit is difficult to design and, if not impossible, would result in the increased manufacturing cost of the cathode ray tube because the magnitude of divergence .DELTA. y will become a high order function of x ang y.
FIG. 27 illustrates, in a schematic perspective representation, the prior art deflection yoke 1 designed to substantially eliminate the occurrence of the seagull distortion shown in FIG. 26. As shown therein, the deflection yoke 1 of the construction shown in and described with particular reference to FIGS. 19 and 20 is additionally provided with a pair of bipolar magnets 10 spaced 180.degree. from each other about the longitudinal axis of the core 2 and secured to an end flange 8a which is fitted to larger diameter end of the core 2 and at a location generally aligned the exit of the electron gun assembly from which the electron beams emerges outwards towards the phosphor-coated screen region. The deflection yoke 1 and FIG. 27 is shown as having a separator 8 mounted on the core 2 in opposition to the end flange 8b.
The operation of the cathode ray tube of the construction shown in FIG. 27 will now be described with reference to FIG. 28. By suitably selecting the shape and the dimensions of each of the bipolar magnets 10, mainly horizontally acting components of the magnetic fluxes 11 developed inside the deflection yoke 1 by the bipolar magnets 10 which emanate from one pole to the opposite pole of each of the bipolar magnets 10 while depicting a loop act on the triads of the electron beams 70a, 70b, 70c and 70d traveling in four diagonal regions so as to deflect the triads of the electron beams 70a to 70d in a direction conforming to the y-axis direction and away from the horizontal axis x as indicated by the arrow 11y, thereby minimizing the seagull distortion 111 which would appear at any one of the four corner areas of the screen that are spaced from the center of the screen in a direction diagonally thereof.
On the other hand, when vertically acting components of the magnetic fluxes 11 developed inside the deflection yoke 1 by the bipolar magnets 10 are considered the triads of the electron beams 70e and 70f which are mainly deflected in the horizontal direction are affected by forces 11x acting along the x-axis so as to draw the triads of the electron beams 70e and 70f close towards each other and in a direction towards the center of the screen and, as a result thereof, the triads of the electron beams 70e and 70f are distorted as shown by 112 in FIG. 29 to represent a generally pincushion distortion with localized protrusions 112a inwardly of the center of the screen. Since such a pincushion distortion tends to be enhanced as a quadratic function as the electron beams traveling towards the phosphor-coated screen region approach the bipolar magnets 10, the use of the conventional pincushion correcting circuit in an attempt to compensate for picture distortion at opposite side portions of the screen would result in that a barrel distortion may be induced in the vicinity of the center of the screen. In addition, the use of the conventional pincushion correcting circuit has an additional problem in that it is ineffective to eliminate the distortion such as represented by the localized protrusions 112a in FIG. 29.
Since the prior art deflection yoke designed to substantially eliminate the seagull distortion occurring in the cathode ray tube having the SP panel is of the construction such as hereinbefore discussed, the pincushion distortion which is a distortion occurring in the horizontal direction tends to be enhanced while the seagull distortion which is a distortion occurring in the vertical direction is substantially minimized. Moreover, the pincushion distortion so developed is not uniform and is complicated in shape accompanied by localized recesses somewhere in the pincushioned picture being reproduced.